Many metal objects and products, such as an automotive vehicle, for example, are subject to corrosion damage and it is desirable to be able to test for the nature and extent of such damage without destroying or impairing the item. Automotive vehicles are subject to supervision and testing of load-bearing parts of the vehicles for corrosion damage are performed periodically by such groups as the German Technical Supervisory Boards. Damages of this kind are in most cases either completely invisible or hard to detect, because they are hidden under protective layers such as paint or corrosion-protection layers, or by dirt. When performing the investigation in the past, scrapers and hammers were used to remove the covering layer so that the surface of the bearing layer was exposed for visual inspection. Light hammer strikes or pressure exerted with a scraper tip allow, to a certain degree, evaluation of the remaining load bearing capability of the investigated sections by an experienced inspector.
From patent application GB No. 2 108 672, a device for detecting previously repaired spots in the sheet steel of vehicle bodies is known. The device consists of a Hall transducer and a magnet mounted so that the magnetic flux of the magnet passes through the transducer into the sheet steel of the vehicle body. Three different light signals, controlled by different levels of the transducer signal voltage indicate (1) whether the sheet steel is separated from the transducer only by a thin paint layer, (2) whether an additional filler layer of non-magnetic material has been applied, and (3) whether there is no ferromagnetic material under the transducer or it is spaced from the transducer by a thick filler layer. Although a device of this kind might be helpful for the mentioned special application, it is generally insufficient, as it delivers information only on the distance between transducer and ferromagnetic material, not, however, on the physical conditions of the latter.
In published German patent application DE-OS No. 2 943 584, a device is described for detecting irregularities of a metal base layer covered by a non-metal cover layer. It operates with an eddy-current sensor placed on the cover layer, the sensor responding on distance variations of the metal base layer. A coil integrated into the sensor forms together with a capacitor a parallel-resonant circuit connected to an oscillator. Reactions of eddy currents in the metal base layer with the coil determine the amplitude of the oscillator voltage. The stronger these reactions are, the smaller the distance is between the coil and base layer. The oscillator voltage accordingly can be considered as a measure of this distance, and can be indicated by a measurement device or, in connection with threshhold switches, by optical signals.
As compared to the unit described previously, the latter patented device has the advantage that it can be used to make measurements even with non-ferromagnetic metals. The disadvantage mentioned in regard to the first described device that, except for the distance to the base layer, no information is obtained, applies here, also.